Method for pretreating wood for the production of cellulose



United States Patent 3 6 Claims. 61. 162-86) The present invention relates to the treatment of wood and more in particular to a method of continuously pretreating wood prior to the sulfite cooking process of the wood with lime milk.

It is well known that wood can be pretreated prior to being subjected to the sulfite cooking process so as to obtain a higher yield of cellulose products having improved parchmentizing properties. According to a known method of pretreatment the wood or wood fibers or other cellulose-containing raw materials arepretreated with an aqueous alkali or alkaline earth substance. The pretreatment is carried out by first adding to the wood fibers, either in the cooking vessel or in another suitable container, several times the quantity of its proper Weight of aqueous alkali or alkaline earth'solutions. The Wood fibers are subjected to the effect of this solution of lye for a period of several hours with a view to impregnating and saturating the Wood as thoroughly as possible with the alkali or alkaline-earth lye. The impregnating process can be further improved by circulating the lye and subjecting the wood fibers to pressure or a vacuum treatment. After the impregnation process the excessive lye is removed. With this known process, an index of saturation of 2.0 can be obtained. By index of saturation, it late be defined as the ratio of the amounts by weight of water contained in a determined amount byweight of theoretically dry wood after the impregnation to the weight of the dry wood. While it is thus possible, according to the known art, to obtain a very thorough impregnation, the impregnation process itself is time-consuming and, in addition, cannot be carried out continuously.

It is therefore the object of the present invention to provide a method for pretreating wood or wood fibers prior to the sulfite cooking process in an entirely continuous manner.

This object is achieved by the method of the present invention which is based on the unexpected discovery that, contrary to common belief, the comparatively small amounts of water received by the wood fibers after an impregnating treatment of only a few minutes are entirely sufiicient to substantially improve the sulfite cooking process in the presence of lime and to greatly increase both the yield and the quality of the parchmentizing properties of the wood fibers. The object of the present invention is thus achieved by adding milk of lime to the Wood fibers and agitating the Wood fibers, the quantity of the milk of lime being so chosen that after the treatment, one weight unit of the theoretically dry wood containsincluding the natural water contents of the woodnot more than one weight unit of water, and then leaving the thus imregnated wood for a predetermined periodand preferably several hours under the influence of the lime.

The milk of lime should contain'an amount of CaO in "the range from 0.5 to of the theoretically dry wood. -Preferably,'the contents are in the'order of l to 2%.

According to another embodiment of the invention the milk of lime is replaced by solutions containing equivalent amounts of alkaline substances or ammonia.

The unexpected and surprising eiiect of the method of the invention will be more fully appreciated on the basis or" the following consideration with reference to the specific test which is, of course, to be considered as an exice ample and not as limitative. Hugged chips of fir wood as conventionally used for producing cellulose and having a Water content of 1 0% are filled into a rotary-cylindrical drum. To the wood fibers are added 0.9 part of milk of lime. The respective indiccs of saturation after various periods of time. which were observed are listed in the 7 following table.

7 Index of Period of time in minutes: saturation 0.5 0.60

It will be seen from the foregoing table that after only one minute a degree of impregnation of 0.65 is obtained, and a degree of impregnationof 0.8 is achieved after six minutes. Thiscomparatively small degree of impregnation is entirely sufficient and results after thesubsequent sulfite cooking process in a considerable increaseof yield and a high improvement of parchmentizing properties.

These results which have been discovered according to the invention and are illustrated herebefore make it impossible to effect a continuous pretreatment with milk of lime. According to the invention this pretreatment can be carried out, for example, by passing hogged chips through a rotating drum and simultaneously adding lime milk. The amount of lime milk added to the hogged chips is adapted to a liquid content of the wood in such a manner that after the treatment, the wood has an 'index of saturation of, for example, 0.8. The pretreatment can be further simplified by providing one or several conveyer belts. The hogged chips are moved by the conveyer belts, and when so moved are sprayed with milk of lime, preferably at those locations where the hogged'chips' are passing from one conveyer belt to another and are thus stirred and well mixed with the lime milk. While the pretreatment with conveyer belts is'very simple, it will be noted that the pretreatment with a rotating drum'is preferable in-as-much as better mixing effect is obtained;

The wood chips thus provided with lime are then stored for at least one hour, or better several hours and thus are subjected to the influence of the lime milk. 'By several hours I wish to be understood a period of about five hours and more, for example two to fifteen hours,

and preferably the storing time is selected from theirange of 5 to 15 hours.

Thereafter, thepretreatment is completed and the sulfite cooking process can be carried out in a known manner. p

In addition to being susceptible to a continuous pretreatment, the method of the invention also offers the considerable advantage of diluting the acid used for the sulfite cooking to a far smaller extentthan :is the case with the known pretreating processes. This will now be explained in further detail.

The known impregnating processes use an excessive amount of milk of lime, and subject the hogged chips to the influencethereof for at least one hour, in the presence of saturation of 2).

pregnated wood chips, can receive only 2.5 'times' the amount by weight of the weight ,ofthe wood, that is only 55% by weight of the normal amount of acid. Consement conditioning the cellulose'yield.

impregnated wood is treated with a cooking acid in a known manner containing 6.3% and 1.1 CaO,"at

pheric pressure of 5.

p v y v.9 j

quently, the acidis diluted by about45 Sinceflthe S0 content of the acid used for the sulfite cooking process is at least 4 to 5%, it becomes necessaryto supply 7.5 to 9% of sulfur dioxide. This is highly disadvantageous because the use of more than 7% of sulfur dioxide makes it necessary to supply liquid 80; whichv renders the process much more, expensive. I I

Contrary to this conventional 'pretreating process the impregnation carried out according to the method of the invention results in an index of saturation, for example, 0.8, so that for one weight unit of wood there can be sup plied to the cooking vessel 3.7 parts by weight of acid; As

. a consequence, acid isdiluted byonly 20%. For that rea son it'is sufiicient to supply only 5 to 6% of sulfur dioxide, which is much-more advantageous than the necessityiof supplying from 7.5 to 9% of S0 as required by the known art, since it is comparatively easy to obtain acid with 5 to 6% I sulfur dioxide at a normal lime content in the order of about 1% CaO anddoes not call forthe supply of liquid sulfurdioxide.

Comparable advantage results are obtained if instead of milk of lime, equivalent amounts of analkaline substance such as, for example, sodium hydroxide or ammonia are added to the hogged chips andare allowed to act upon the latter forat least one hour and preferably several hours.

r A further advantageous feature 'of the invention resides in the fact that the wood chips are subjected to the influence of the milk oflime fora considerable period of time, for example at least one hour, and preferably 5 to 15 hours. .It has been discovered and it will become further apparent from the following examples that the cellulose yield after the sulfite cooking subsequent to the impregnation carried out according to the invention rises COHSldl'flf bly in direct proportion to an increase of the period. of

time in which the wood chips are influenced by themilk of lime. 7

Example I Hoggedchips of fir wood having a liquid content of 10% are filled into a rotating cylindrical vessel. A milk of lime is prepared containing 12.5 grams of CaO per liter and 0.7 part by weight, with referenceto the absolutely theoretically dry weight of the. wood, is supplied to the Wood chips. After minutes the .liquid is completely increase in yield of cellulose.

Example II g Example'pl. is repeated with the exception, that the milk of lime contains 25grarns CaOper liter and the im pregnated Wood contains 2.0% CaO. The sulfite cooking is effected with an. acid containing '5.5%' S0 and 1.1% CaO during 9 /1 hours at a super-atmospheric 'pressure 0 55 anda maximumtemperature of 129 C.

The following resultsare obtained, conditioned upon 5 the following various. periods of duration of the-action It will be noted from the foregoingtable that the lime must be allowed to act on the wood chips for a period of preferably 10-12 hours in order to get the maximum The foregoing examples also clearly demonstrate that the increase in yield of cellulose further depends on the amount of lime absorbed absorbed bythe wood chips, the latter, having an index of saturation of 0.8 and containing 1.0% CaO.

The thusimpregnated wood chips are left'under. the

influence of the lime at room temperatureyfor.apredetermined period of time. A series of tests is made with the period of time ranging from 0 to 16 hours, as indicated in the reference table below, the various lengths of treat- Thereafter the a maximum temperature of 127 C; and a super-atmos- I The following results are obtained, conditioned upon the following various periods of duration of the-action of the 111116 on the hogged chips:

, Additional Degrce of Cellulose yield with dceomposlyield in reference 1 tion percent of to un- Duration of action of lime (Johnson, wood treated N 011- wood in number) percent of weight of.

Untreated 45 53.7 1 hour... 46 55. l 2.6 46. 5 56.0 4. 3 46 56. 2 4. 7 47 56. 4 5. 0 4.7 56. 7 5. 6 l9 56.8 5; 8

by the -wood chips; for example, where 1% ICaO is contained in the wood chips, the increase is 5 6%, whereas the increase rises to 78% if the CaO 'contentof the wood chips is 2% 5 The effect of the-further increase of'the'CaO content is'demonstrated by the-following Example III.

Example 111 Example I is repeated with the-exception that the wood is treated with a milkof limecontaining per liter 37.5 grams CaO. After the impregnation, the wood contains 3.1% CaO; It is then stored for a period of twelve hours, during which time the lime is allowed to act'on the Wood chips, whereupon the Wood chips are digested with an acid containing :6% S0 and 0.84%

CaO for aperiod of 9 hours and a maximum temperature of, 126 C. The super-atmospheric pressure is ,5. The yield is 65.7% of the weightof the wood and. a cellulose is obtained with ,a decomposition degree (Johnsen-Noll number 73)..

. If. the .sulfite. cooking is effected with untreated wood,

the yield is only 5 8.8% and the J ohnsen-Noll number is only 70. The pretreatmentwith 3% C210 thus results vin a yield increase of 12% "of the cellulose weight. I

It'will be noted that the cellulose material obtained after a pretreatment according IOiihedIlVfilltiOD, and in.

particular the cellulose material obtained after the pregtreatnrentdescribed in Examples'l to ,3, has also greatly increased parchmentizing properties. It has been'found that there is a certainiproportionality between the increase of the yield and the improvement, of the parchmentizing property. Withother-wordawhere, as a consequence of the pretreating method of the inventiomthe cellulose yield has increased, the parchmentizing properties have also improved, vwiththe latter;i mprovement.1being so much more pronounced as. the yield increaseis higher.

Emma 1 W Example I is repeated with-the exception that for the impregnation of the hogged chips, the lime is replaced by z, 0.7 part by Weight of-sodium hydroxide containing -35 grams sodium hydroxide per liter. *The Wood has an index of saturation of 0.8 and after theimpregnation contains 0.8% of sodium hydroxide; .Afterthe sulfite cooking according to Example I the sodium hydroxide is allowed to act upon the wood for a period of 12 hours. The degree of decomposition (Johnsen-Noll number) is 62, and the cellulose yield of 58.5% of the Wood is obtained. In a treatment without impregnation the yield is only 53.5%.

The additional yield with reference to untreated wood in percent of the weight of the pulp is 9.0.

Example V Example I is repeated with the exception that for the impregnation of the hogged chips the lime is replaced by 0.7 part by weight of an ammonia solution containing 15 grams of ammonia per liter. The impregnation degree of the wood is 0.8, and after the impregnation it contains 1.2% ammonia (NH Thereafter the ammonia is allowed to act upon the hogged chips in a period of 12 hours, whereupon the sulfite cooking is effected according to Example I. The cellulose yield (in percent) of the wood is 58.0%, the Iohnsen-Noll number is 50.

After an analogous processing without impregnation treatment the cellulose yield is only 53.1%; the Johnsen- Noll number is 48.

The additional yield with reference to untreated wood in percent of the weight of the pulp is 9.0

It will be understood that this invention is susceptible to further modification and, accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claims.

What I claim is:

1. In a process for the production of cellulose from fir wood comprising first pretreating the Wood by impregnation with an alkaline solution and then subjecting said pretreated wood to a sulfite digestion, the improvement which comprises discontinuing said impregnation when up to an amount by weight of the alkaline solution is absorbed in the wood, so that one part by weight of the theoretically dry wood contains at the end of the impregnation not more than one part by weight of water including the natural water content of the wood, the strength of said alkaline solution being equivalent to an amount of 0210 in the range of 0.5-5 of the theoretically dry wood; and storing said impregnated wood for at least one hour prior to introducing said wood into the sulfite digestion.

2. The process of claim 1, wherein the alkaline solution comprises a member of the group consisting of milk of lime, sodium hydroxide, and ammonia.

3. The process of claim 1, wherein the impregnated wood is stored for at least five hours.

4. The process of claim 1, wherein the impregnated wood is stored for 5-15 hours.

5. The process of claim 1, wherein the ratio of water to dry wood is about -100 parts water to parts wood.

6. The process of claim 3, wherein the ratio of water to dry wood is about 80-100 parts water to 100 parts wood.

References Cited by the Examiner UNITED STATES PATENTS 1,857,695 5/32 Rosen 162-90 1,885,764 11/32 Richardson l62-86 2,041,745 5/ 36 Dreyfus l6286 2,610,119 9/52 Magnuson 162-17 FOREIGN PATENTS 526,399 9/ 40 Great Britain.

DONALL H. SYLVESTER, Primary Examiner.

RICHARD D. NEVIUS, MORRIS O. WOLK, Examiners. 

1. IN A PROCESS FOR THE PRODUCTIN OF CELLULOSE FROM FIR WOOD COMPRISING FIRST PRETREATING THE WOOD BY IMPREGNATION WITH AN ALKALINE SOLUTION AND THEN SUBJECTING SAID PRETREATED WOOD TO A SULFITE DIGESTION, THE IMPROVEMENT WHICH COMPRISES DISCONTINUING SAID IMPREGNATION WHEN UP TO AN AMOUNT BY WEIGHT OF THE ALKALINE SOLUTION IS ABSORBED IN THE WOOD, SO THAT ONE PART BY WEIGHT OF THE THEORETICALLY DRY WOOD CONTAINS AT THE END OF THE IMPREGNATION NOT MORE THAN ONE PART BY WEIGHT OF WATER INCLUDING THE NATURAL WATER CONTENT OF WOOD, THE STRENGTH OF SAID ALKALINE SOLUTION BEING EQUIVALENT TO AN AMOUNT OF CAO IN THE RANGE OF 0.5-5% OF THE THEORETICALLY DRY WOOD; AND STORING SAID IMPREGNATED WOOD FOR AT LEAST ONE HOUR PRIOR TO INTRODUCING SAID WOOD INTO THE SULFITE DIGESTION. 